I. Field of the Invention
This invention relates generally to an article of manufacture and more specifically to a tactile layer which may be embodied in a diaphragm-type switch keyboard to impart a tactile signal to the user that a switch closure has been made. Keyboards of the type described find wide application in a variety of devices including hand calculators, remote TV tuners, computer I/O devices, etc.
II. Discussion of the Prior Art
Diaphragm-type switches are well known in the art. They generally comprise first and second spaced apart contacts, one of the contacts being disposed on a flexible membrane. When force is applied to the membrane it deforms, sometimes through an apertured spacer layer, to establish an electrical contact. Typical of this arrangement is that shown in the Krakinowski U.S. Pat. No. 3,308,253 and the Kaelin et al. U.S. Pat. No. 3,732,389. The foregoing arrangements are exemplary of so-called non-tactile keyboards wherein depression of the membrane does not provide a noticeable impulse to the finger of the user upon switch actuation.
Tactile-type diaphragm switches are also known in the art. For example, reference is made to the published German patent application No. 1,806,241, published Aug. 14, 1969 which teaches the use of a metallic dome shaped projection which when depressed, provides a noticeable snap as the dome is inverted to establish the electrical contact. When the finger pressure is removed, the dome is restored to its normal, non-inverted configuration. This arrangement is commonly referred to as incorporating the "oil-can" effect. Another example of this arrangement is set forth in the IBM Technical Disclosure Bulletin, Volume 7, No. 12, dated May 1965, only here, rather than a metallic dome, a plastic membrane having a dimpled surface is employed.
The Lynn et al. U.S. Pat. No. 3,860,771 and the Adams et al. U.S. Pat. No. 3,246,112 each discloses a tactile keyboard arrangement in which the tactile layer comprises a sheet of flexible plastic material having integrally formed depressions comprising a generally cylindrical pedestal having straight or sloping walls and topped or capped by a portion of a curved surface, such as a portion of a sphere. Again, as the dome is inverted, the user is provided with a snap action indicative of a contact closure.
The tactile layer of the present invention is deemed to be an improvement over the aforementioned prior art arrangements in that it provides a keyboard arrangement that is not subject to "edge toggling". As is set forth in my co-pending patent application Ser. No. 841,981, filed Oct. 13, 1977 and entitled "TACTILE TOUCH SWITCH PANEL", devices made in accordance with certain aspects of the prior art suffer from a defect which may be termed "edge toggle". Edge toggle occurs when only one portion of the tactile dome collapses, or when one portion of the dome collapses late and produces a double tactile feedback sensation. This edge toggle always occurs along a crease line where the slope of the crease's center wall approaches the vertical. Five characteristics of edge toggling may be observed and are as follows:
1. The action is not concentric and proceeds from the center of the dome to only one segment of the outer circumference thereof; PA1 2. The collapse of the dome is not catastrophic and does not always go to completion; PA1 3. Movement of the flexible dome material is not always isolated within the dome and tends to lift the surrounding circuit layer from its substrate; PA1 4. The tactile feedback sensation is very dependent upon the location on the dome where the force is applied; and PA1 5. The tactile feedback is not consistent and may be different every time the dome is collapsed.
In addition to the undesirable edge toggling effect, it has also been found that certain keyboards having simple domes in their tactile layer tend to show a substantial variation in "feel" depending upon the location of the key on the keyboard. More specifically, when the keys are arranged in rows and columns and where simple domes are employed such as in the aforereferenced IBM Technical Disclosure Bulletin, one can easily detect a variation in feel between the keys located at the corners of the array than that which is obtained from keys which are surrounded on all four sides by other key locations. In the same fashion, still a different "feel" is observed at key locations along the edges of the array, but which are not corner locations. This variation in "feel" is, of course, objectionable.
In accordance with the teachings of the Lynn et al Patent referred earlier, in order to obtain a desired feel, the use of an apertured spacer disposed between the tactile layer and the circuit bearing layer is recommended. This additional spacer layer naturally increases the complexity and attendant cost of a keyboard.